Shell ejecting air gun?

Im making an marble air gun just like the one the teach you how to do at sentex.net and i was wondering if anyone could give drawings on how to make the marble gun eject shells?
Is it possible on a home air gun?
Please tell me beucause it would be really nice if i could eject shells on my homemade marble gun.
And not only that but another question as well: Can i make bullets myself from melted tin?